Synthesis of novel rhodium-carbene complexes as efficient catalysts for addition of phenylboronic acid to aldehydes

Article abstract of DOI:10.1016/j.molcata.2003.12.037

Rhodium(I)-carbene complexes with 1,3-dialkylimidazolidin-2-ylidine ligand were prepared and characterized by conventional spectroscopic methods and elemental analyses. The rhodium-carbene complexes RhCl(COD)(1,3-dialkyl- imidazolinylidene), 2a-d, are an

Full text of DOI:10.1016/j.molcata.2003.12.037

Journal of Molecular Catalysis A: Chemical 215 (2004) 45–48
Synthesis of novel rhodium–carbene complexes as efficient
catalysts for addition of phenylboronic acid to aldehydes
Ismail Özdemira,∗, Serpil Demira, Bekir Çetinkayab
a
Department of Chemistry, Inönü University, 44069 Malatya, Turkey
b
˙
Department of Chemistry, Ege University, 35100 Bornova-Izmir, Turkey
Received 23 September 2003; accepted 16 December 2003
Abstract
Rhodium(I)–carbene complexes with 1,3-dialkylimidazolidin-2-ylidine ligand were prepared and characterized by conventional spectro-
scopic methods and elemental analyses. The rhodium–carbene complexes RhCl(COD)(1,3-dialkyl-imidazolinylidene), 2a–d, are an active
catalyst for addition of phenylboronic acid to aldehydes.
© 2004 Elsevier B.V. All rights reserved.
Keywords: N-Heterocyclic carbene; Rhodium complexes; Imidazolidinylidine; Arylation; Aldehydes
1. Introduction
rhodium and palladium catalysts, which are less oxygen
sensitive in comparison to the phosphine analogues [10].
Our contribution to this field has started with syntheses of
imidazolidin-2-ylidenes complexes of Rh(I) and Ru(II)
which are capable of catalyzing the cyclopropanation of sty-
rene with ethyl diazoacetate and intramolecular cyclization
of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-dimethylfuran
in good yields [4,5]. These complexes are insensitive to air
and moisture and are thermally stable in both the solid state
and in solution. We have now considered the possibility of
generating heterocyclic carbenes that have pendent arene
group to evaluate their ability to chelate the ruthenium
atom, stabilise the complex or create catalytic activity [11].
Rhodium–carbene complexes have been extensively stud-
ied. However, there are few reports on the catalytic activity
of rhodium–carbene complexes in rhodium-mediated pro-
cesses [9,12]. Miyaura and co-workers [13] reported that
rhodium catalyzes the addition of aryl and alkenylboronic
acids to aldehydes giving secondary alcohols. The reactions
were facilitated by the presence of an electron withdrawing
group on the aldehyde and an electron donating group on the
arylboronic acid, suggesting that the mechanism involves a
nucleophilic attack of the aryl group on the aldehyde. The
finding that these reactions were run with sterically hindered
and strongly basic ligands attracted the attention of Fürst-
ner who subsequently applied N-heterocyclic carbene lig-
ands. A in situ generated catalytic system for the addition
In the last decade N-heterocyclic carbenes (NHC’s) have
been the subject of intense research in the field of organo-
metallic chemistry [1,2]. Because of their extraordinary
properties they have found access to a great variety of
catalytic processes which include C–C-coupling reactions,
[3] formation of furans, [4] cyclopropanation, [5] olefin
metathesis, [6] hydroformylation, [7] polymerization [8]
and hydrosilylation reactions [9].
Complexes containing imidazolin-2-ylidenes, which are
neutral two-electron-donor ligands with negligible ␲-back-
bonding, are thermally rather stable. This feature represents
an essential prerequisite for the synthesis of highly efficient
catalysts. The low reactivity of these ligands as compared
to Fischer and Schrock type carbenes and to tertiary phos-
phines, which are susceptible to facile oxidation or P–C
degradation makes them useful ancillary ligands to control
the reactivity in a number of catalytic processes. Employ-
ment of sterically demanding N-heterocyclic carbenes,
which resemble bulky phosphines with respect to their
bonding, has led to the synthesis of new robust ruthenium,
∗
Corresponding author. Fax: +90-422-3410037.
E-mail address: iozdemir@inonu.edu.tr (I. Özdemir).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.12.037

46
I. Özdemir et al. / Journal of Molecular Catalysis A: Chemical 215 (2004) 45–48
2.3. RhCl(COD)[CN{CH2C6H2Me3 − 2, 4, 6}CH2CH2N
{CH2C6H2Me3 − 2, 4, 6}] (2a)
Scheme 1.
1H NMR (δ, CDCl3): 2.83 [s, 4H, NCH2CH2N];
6.80 [s, 4H, CH2C6H2Me3-2,4,6]; 5.00 and 3.60 [s, 4H,
CH2C6H2Me3-2,4,6]; 2.33 and 2.19 [s, 18H, CH2C6H2Me3-
2,4,6]; 5.50 and 5.03 [d, 3J = 13.90 Hz, 4H, CHCOD];
2.37 and 1.9 [m, 8H, CH2COD]; 13C {H}NMR (δ, CDCl3):
214.6 [d, J = 46.58 Hz, Ccarbene]; 47.7 [NCH2CH2N];
138.6, 138.0, 129.6 and 129.4 [CH2C6H2Me3-2,4,6]; 49.1
[CH2C6H2Me3-2,4,6]; 21.3 and 21.1 [CH2C6H2Me3-2,4,6];
99.5 and 68.0 [d, J = 6.64 Hz and J = 14.65 Hz, CHCOD];
33.3 and 29.1 [CH2COD]. Yield 0.540 g (93%).
of phenylboronic acid to aldehydes is prepared combination
of rhodium salt, 1,3-bis(alkyl or aryl)imidazolium chloride
and base [14].
Based on these findings and our continuing interest in
developing more efficient and stable catalysts, we wished to
examine whether we could influence the catalytic activity of
rhodium complexes for the addition of phenylboronic acid
to aldehydes (Scheme 1).
We now report (i) the straightforward preparation of new
RhCl(COD)(1,3-dialkyl-imidazolinylidene) complexes; (ii)
their efficiency as catalysts for the addition of phenylboronic
acid to aldehydes.
Anal. cal. for C31H42N2ClRh; C: 64.08, H: 7.28, N: 4.82;
found C: 64.11, H: 7.33, N: 4.86.
2.4. RhCl(COD)[CN{CH2C6H2(OMe)3−3,4, 5}CH2CH2N
{CH2C6H2(OMe)3 − 3, 4, 5}] (2b)
2. Experimental
1H NMR (δ, CDCl3): 3.24 [s, 4H, NCH2CH2N];
6.79 [s, 4H, CH2C6H2(OMe)3-3,4,5]; 4.98 and 3.37 [s,
4H, CH2C6H2(OMe)3-3,4,5]; 3.82 and 3.77 [s, 18H,
Manipulations were prepared with standard Schlenk
techniques under an inert atmosphere of nitrogen with pre-
viously dried solvents. The complex [RhCl(COD)]2 [18]
and 1,3-dialkyl-imidazolinylidene were prepared according
to known methods [15]. Infrared spectra were recorded as
KBr pellets in the range 400–4000 cm−1 on a ATI UNI-
CAM 2000 spectrometer. 1H NMR (300 MHz) and 13C
NMR (75.5 MHz) were recorded on a Bruker AM 300 WB
FT spectrometer with chemical shifts referenced to resid-
ual solvent CDCl3. Microanalyses were performed by the
3
CH2C6H2(OMe)3-3,4,5], 5.97 and 4.55 [d, J = 14.22 Hz,
4H, CHCOD]; 2.32 and 1.89 [m, 8H, CH2COD]; 13C
{H}NMR (δ, CDCl3): 213.7 [d, J = 47.3 Hz, Ccarbene];
48.3 [NCH2CH2N]; 154.8, 138.7, 133.1 and 106.7
[CH2C6H2(OMe)3-3,4,5], 61.6 [CH2C6H2(OMe)3-3,4,5],
57.2 and 55.6 [CH2C6H2(OMe)3-3,4,5], 100.5 and 69.4 [d,
J = 6.40 Hz and J = 14.40 Hz, CHCOD], 33.5 and 29.3
[CH2COD]. Yield 0.602 g (89%).
Anal. cal. for C31H42N2O6ClRh; C: 54.99, H: 6.25, N:
4.14; found C: 55.03, H: 6.24, N: 4.16.
˙
TÜBITAK analyses centre.
2.1. General procedure for the preparation of the
rhodium–carbene complexes (2a–d)
2.5. RhCl(COD)[CN{C5H9}CH2CH2N{C5H9}] (2c)
1H NMR (δ, CDCl3): 3.4 [m, 4H, NCH2CH2N]; 4.9 [s,
2H, CHCp]; 1.8 [m, 24H, CH2CpandCOD]; 5.7 and 5.6 [d, 3J =
8.0 Hz, 4H, CHCOD]; 13C {H}NMR (δ, CDCl3): 211.1 [d,
J = 46.5 Hz, Ccarbene], 42.9 [NCH2CH2N]; 61.4 [CHCp];
29.2, 28.9, 24.9 and 24.6 [CH2Cp]; 98.4 and 68.2 [d, J =
6.9 Hz and J = 14.6 Hz, CHCOD]; 33.3 and 29.9 [CH2COD].
Yield 0.390 g (86%).
A solution of 1,3-dialkyl-imidazolinylidene (1) (0.5 mmol)
and [RhCl(COD)]2 (0.5 mmol) in toluene (15 ml) was
heated under reflux for 2 h. Upon cooling to room temper-
ature, yellow-orange crystals of 2a–d were obtained. The
crystals were filtered, washed with diethyl ether (3 × 15 ml)
and dried under vacuum.
Anal. cal. for C21H35N2ClRh; C: 55.57, H: 7.77, N: 6.17;
found C: 55.61, H: 7.74, N: 6.19.
2.2. General procedure for rhodium–carbene catalyzed
addition of phenylboronic acid to aldehydes
Phenylboronic acid (1.20 g, 9.8 mmol), KOtBu (4.9 mmol),
substituted aldehydes (4.9 mmol), rhodium carbene catalyst
(1 mol%), dimethoxyethane (15 ml) were introduced in to
Schlenk tube and then water (5 ml) was added. The result-
ing mixture was heated for 0.4–2.0 h at 80 ◦C, cooled to
ambient temperature, extracted with ethyl acetate (30 ml).
After drying over MgSO4 the organic phase was evapo-
rated and the residue was purified by flash chromatography
(hexane/ethyl acetate, 6/1).
2.6. RhCl(COD)[CN{CH2C6H2(OMe)3−3,4,5}CH2CH2N
{CH2CH2OMe}] (2d)
1H NMR (δ, CDCl3): 4.12 and 3.41 [s, 4H, NCH2CH2N];
6.75 [s, 2H, CH2C6H2(OMe)3-3,4,5]; 4.92 [s, 2H, CH2C6
H2(OMe)3-3,4,5]; 3.83 and 3.77 [s, 9H, CH2C6H2(OMe)3-
3,4,5]; 3.69 [m, 2H, CH2CH2OCH3]; 4.47 [m, 2H,
CH2CH2OCH3]; 3.33 [m, 2H, CH2CH2OCH3]; 5.90 and
4.53 [d, 3J = 7.2 Hz, 4H, CHCOD]; 2.29 and 1.85 [m,

I. Özdemir et al. / Journal of Molecular Catalysis A: Chemical 215 (2004) 45–48
47
8H, CH2COD]; 13C {H}NMR (δ, CDCl3): 213.2 [d, J =
46.7 Hz, Ccarbene]; 50.71 and 48.08 [NCH2CH2N]; 153.9,
137.8, 132.4 and 106.1 [CH2C6H2(OMe)3-3,4,5]; 61.24
[CH2C6H2(OMe)3-3,4,5]; 57.0 and 55.3 [CH2C6H2(OMe)3-
3,4,5]; 68.7 [CH2CH2OCH3], 72.5 [CH2CH2OCH3]; 59.3
[CH2CH2OCH3]; 99.6 and 69.1 [d, J = 7.85 Hz and J =
10.85 Hz, CHCOD]; 33.3 and 29.2 [CH2COD]. Yield 0.499 g
(90%).
Table 2
Rhodium–carbene catalyzed addition of phenylboronic acid to aldehydes
Entry
Catalyst
R
Time (h)
Yield (%)a,b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2a
2b
2c
2d
2a
2a
2b
2c
2d
2a
2a
2b
2c
2d
2a
2a
2b
2c
2d
2a
p-OCH3
p-OCH3
p-OCH3
p-OCH3
0.5
0.5
1.5
0.4
1.0
0.5
0.5
2.0
0.5
1.0
0.5
0.5
1.5
0.5
2.0
0.5
1.0
2.0
0.5
2.0
91
89
85
96
75d
95
97
82
94
73d
91
93
81
90
77d
91
86
74
89
71d
p-OCH3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
3,4,5-(OCH3)3
3,4,5-(OCH3)3
3,4,5-(OCH3)3
3,4,5-(OCH3)3
3,4,5-(OCH3)3
p-Cl
Anal. cal. for C24H36N2O4ClRh; C: 51.95, H: 6.54, N:
5.05; found C: 51.96, H: 6.55, N: 5.10.
3. Results and discussion
The tetraaminoethene, 1, was synthesised using a method
similar to that reported by Çetinkaya et al. [15]. The reaction
of tetraaminoethene 1 with the binuclear [RhCl(COD)]2
complex proceeded smoothly in refluxing toluene to give
the RhCl(COD)(1,3-dialkyl-imidazolinylidene) complexes
2a–d as crystalline solids in 86–93% yields (Scheme 2,
Table 1).
Complexes 2a–d, which are very stable in the solid state
have been characterized by analytical and spectroscopic
techniques (Table 1). Rhodium complexes exhibit a charac-
teristic υ(NCN) band (Table 1) typically at 1590–1612 cm−1
[5,16]. 13C chemical shifts, which provide a useful diag-
nostic tool for metal–carbene complexes, show that Ccarb
is substantially deshielded. Values of δ(13Ccarb) are in the
range 211–214 ppm and are similar to those found in other
carbene complexes. Coupling constants J(103Rh–13C) for
the new rhodium complexes (2a–d) are comparable with
those found for carbene rhodium(I) complexes (Table 1).
These new complexes show typical spectroscopic sig-
natures which are in line with those recently reported
for other RhCl(COD)(1,3-dialkyl-imidazolinylidene) com-
plexes [5,16,17].
p-Cl
p-Cl
p-Cl
p-Cl
a
Isolated yield (purity of yield checked by NMR).
Yields base on the aldehydes.
All reactions were monitored by TLC.
Base K2CO3.
b
c
d
Although the addition of carbon nucleophiles to aldehy-
des is usually a facile process, limits are encountered that
functionalized organometallic reagents required. Rhodium
complexes 2a–d were found to be active catalysts for the
addition of phenylboronic acid to aldehydes and proved to
be thermally robust for high temperature. The addition of
phenylboronic acid to aldehydes proceeds in high yields
and quite rapidly even with a low catalyst loading. The re-
sults were summarized in Table 2. Under those conditions,
4-methoxybenzaldehyde,
2,4,6-trimethylbenzaldehyde,
3,4,5-trimethoxybenzaldehyde and 4-chlorobenzaldehyde
react very cleanly with phenylboronic acid in goods yields
(Table 2, entries 4, 7, 12 and 16).
R
N
R'
N
R
N
1/2
1/2
+
[RhClCOD]2
Rh
Cl
N
N
R
4. Conclusion
N
R'
R'
1
2
From readily available starting materials, such as
1,3-dialkyl-imidazolinylidene four rhodium–carbene (2a–d)
have been prepared and characterized. Also a convenient
and highly user-friendly method for the addition of phenyl-
boronic acid to aldehydes is presented. The procedure is
simple and efficient towards various aryl aldehydes and
does not require induction periods.
a : R = R' = CH2C6H2Me3-2,4,6
b : R = R' = CH2C6H2(OMe)3-3,4,5
c : R = R' = C5H9
d : R = CH2CH2OMe; R' = CH2C6H2(OMe)3-3,4,5
Scheme 2.
Table 1
Selected analytical data for the new rhodium–carbene complexes (2a–d)
Complex
Isolated
yield (%)
mp (◦C)
ν(NCN)
(cm−1
)
13C NMR
Rh-C δ ppm
(1JRh-C)
Acknowledgements
2a
2b
2c
2d
93
89
86
90
235.0–235.5
218.0–219.0
221.5–222.0
152.5–153.0
1612
1592
1590
1593
214.6 (46.6 Hz)
213.7 (47.3 Hz)
211.1 (46.5 Hz)
213.2 (46.7 Hz)
We thank to Technological and Scientific Research Coun-
cil of Türkiye TÜBITAK (TÜBITAK COST D17) and Inönü
University Research Fund for financial support of this work.
˙
˙

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[7] W.A. Herrmann, J.A. Kulpe, W. Konkol, H. Bahrmann, J. Organomet.
References
Chem. 389 (1990) 85–101.
[8] M.G. Gardiner, W.A. Herrmann, C.-P. Reisinger, J. Schwarz, M.
Spiegler, J. Organomet. Chem. 572 (1999) 239–247.
[9] (a) M.F. Lappert, R.K. Maskell, J. Organomet. Chem. 264 (1984)
217–228;
(b) J.E. Hill, T.A. Nile, J. Organomet. Chem. 137 (1977) 293–300.
[10] (a) M. Scholl, S. Ding, C.W. Lee, R.H. Grubbs, Org. Lett. 1 (1999)
953–956;
[1] (a) F.Z. Dörwald, Metal Carbenes in Organic Synthesis, Wiley-VCH
Verlag GmbH, Weinheim, 1999, pp. 1–31;
(b) K.H. Dötz, H. Fischer, P. Hofmann, F.R. Kreisse, U. Schubert,
K. Weiss, Transition Metal Carbene Complexes, Verlag Chemie,
Weinheim, 1983, pp. 31–226;
(c) D. Bourissou, O. Guerriet, P.F. Gabbai, G. Bertrand, Chem. Rev.
100 (2000) 39–92.
(b) S.B. Garber, J.S. Kingsbury, B.L. Gray, A.H. Hoveyda, J. Am.
Chem. Soc. 122 (2000) 8168–8179;
(c) S. Gressler, S. Randl, S. Blechert, Tetrahedron Lett. 41 (2000)
9973–9976.
[2] (a) D.J. Cardin, B. Çetinkaya, M.F. Lappert, Lj. Manojlovic-Muir,
K.W. Muir, J. Chem. Soc. Chem. Commun. (1971) 400–401;
(b) M.F. Lappert, J. Organomet. Chem. 358 (1988) 185–214.
[3] (a) V.P.W. Böhm, T. Weskamp, C.W.K. Gstöttmayr, W.A. Herrmann,
Angew. Chem. Int. Ed. 39 (2000) 1602–1604;
[11] (a) B. Çetinkaya, S. Demir, I. Özdemir, L. Toupet, D. Sémeril, C.
Bruneau, P.H. Dixneuf, New J. Chem. 25 (25) (2001) 519–521;
(b) B. Çetinkaya, S. Demir, I. Özdemir, L. Toupet, D. Sémeril, C.
Bruneau, P.H. Dixneuf, Chem. Eur. J. 9 (2003) 2323–2330.
[12] A.C. Chen, L. Ren, A. Decken, C.M. Crudden, Organometallics 19
(2000) 3459–3461.
[13] M. Sakai, M. Ueda, N. Miyaura, Angew. Chem. Int. Ed. 37 (1998)
3279–3281.
[14] A. Fürstner, H. Krause, Adv. Synth. Catal. 343 (2001) 343–350.
[15] E. Çetinkaya, P.B. Hitchcock, H.A. Jasim, M.F. Lappert, J. Chem.
Soc. Perkin Trans. 1 (1992) 561–567.
[16] B. Çetinkaya, P.B. Hitchcock, M.F. Lappert, D.B. Shaw, K. Spy-
ropoulos, N.J.W. Warhurst, J. Organomet. Chem. 459 (1993) 311–
317.
[17] I. Özdemir, B. Yig˘it, B. Çetinkaya, D. Ülkü, M.N. Tahir, C. Arıcı,
J. Organomet. Chem. 633 (2001) 27–32.
(b) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290–
1309.
[4] (a) B. Çetinkaya, I. Özdemir, C. Bruneau, P.H. Dixneuf, J. Mol.
Catal. A: 118 (1997) L1–L4;
˙
(b) I. Özdemir, B. Yig˘it, B. Çetinkaya, D. Ülkü, M.N. Tahir, C.
Arıcı, J. Organomet. Chem. 633 (2001) 27–32;
(c) B. Çetinkaya, T. Seçkin, N. Gürbüz, I. Özdemir, J. Mol. Catal.
A: 184 (2002) 31–38.
[5] B. Çetinkaya, I. Özdemir, P.H. Dixneuf, J. Organomet. Chem. 534
(1997) 153–158.
[6] (a) C.W. Bielawski, R.H. Grubbs, Angew. Chem. Int. Ed. 39 (2000)
2903–2906;
(b) A.K. Chatterjee, J.P. Morgan, M. Scholl, R.H. Grubbs, J. Am.
Chem. Soc. 122 (2000) 3783–3784;
(c) T.M. Trnk, R.H. Grubbs, Acc. Chem. Res. 34 (2001) 18–29;
(d) A. Fürstner, Angew. Chem. Int. Ed. 39 (2000) 3012–3043.
[18] J. Chatt, L.M. Venanzi, J. Chem. Soc. (1975) 4735–4736.